Geology in space

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Previous posts have looked at the nature of tectonics on the terrestrial planets and how they are very different to Earth. Further out in the solar system, it is not the planets which are of geological interest but the moons, many of these are ice worlds. Cores of rock covered in layers of ice. Some of these have pockets or layers of water in them generated by tidal friction generating heat.

Evidence for this layer of water or at least an icy slush is shown by fountains of ice sprayed into space on both Enceladus and Europa showing that these moons are active places however, there are other telltale signs of a system similar to plate tectonics.

Photos of Europa, an Ice moon of Jupiter, revealed a cracked surface covered in bands. Limited cratering suggests that surface is relatively young (around 40-90 million years). Some of the surface shows repeating bands similar to spreading ridges seen on Earth. Where the surface ice cracks and moves apart and new water rises up and fills the gap with ice.

Europa as seen by Galileo (Nasa/JPL-Caltech/SETI institute)

These same spreading ridges and associated bands are also clearly seen on Miranda, a moon of Uranus, which has alternating patterns around a central rift, just like is seen on the ocean plates of Earth.

Where tectonics must differ is what happens at the other end; ice is very buoyant making it difficult to find a form of subduction which would allow pulling of the crust down into the water/ slush layers like the plates on Earth. As the surfaces of these moons cannot be ever increasing in diameter, something has to be happening to the ice.

On Europa there no regional scale mountain belts where the crust thickens to accumulate this extra crust. Instead, there is some evidence of complex fault systems. Similar to subduction zones, but which behave differently and have been termed subsumption zones – one section of the ice crust is forced under another, where it rapidly becomes incorporated into the underlying water ice… so rather than continuing to sink like the plates on Earth it rapidly becomes part of the underlying layers, enabling tectonics to continue.

Miranda as seen my Voyager 2 (Nasa/JPL)

The ice moons of the outer solar system are harsh alien places, but tectonically they may be the most Earth-like of all the bodies in the solar system.

Last time I talked about the different ways rocks breakdown in the space environment. This time we will look at what happens to some of these products of the erosion and weathering of the materials go.

Whilst many of the smaller bodies in the solar system don’t have an atmosphere (which would stop most space weathering), there is still atoms/ions/dust particles floating around above the surface. Whilst this material is too spread out to interact with each other in the way that a gas does in an atmosphere, these materials are still trapped, bound by the gravity of the body – this is called the exosphere.

Unlike other bodies, these exospheres are predominantly comprised of material generated from space-weathering; sputtering releasing atoms, and melting which releases volatile molecules. Some are released from surfaces by heating at sunrise. As the solar wind of charged particles moves over a body, its surface can become charged, dust particles gain a similar charge and so are repelled floating above the surface. The amount of dust is linked to the number of micrometeorite impacts, the intensity of the solar wind, and any magnetic fields. This dust can refract light; giving the appearance of a diffuse sunrise which would otherwise not be possible on an airless world.

Lunar Horizon Glow on the Moon, generated by dust in its exosphere, taken by Nasa’s Clementine mission (NASA)

For rocker bodies, such as Mercury and the Moon atoms of Ca, Mg, & K have been detected. For some of the other bodies in the solar system, beyond the frost line – water ice covers many of the bodies like Europa and Enceladus, when the surfaces are broken down by radiation, the hydrogen escapes, leaving oxygen and hydroxide around them.

The Calcium and Magnesium tails of Mercury as taken by MESSANGER (NASA/Johns Hopkins/ Carnegie institution

These materials don’t stay in the exosphere forever, s0me can be reabsorbed by the surface of the planet, often on the night side of a planet, and the dust and atoms can settle down. Atoms can become charged by sunlight (photo-ionisation), or interaction with charged particles in the solar wind. These ions are caught by solar winds or by magnetic fields and expelled out into interplanetary space or fired at high speed into the ground close to the poles. The force of sunlight hitting atoms or the rare collisions between atoms can slowly add momentum until some material eventually gain enough velocity and can escape the exosphere.